Danish Patent No. 149238 discloses a zero-flux current transformer for measuring the current for an electromagnet in a particle accelerator. As very high current values are involved, viz. several hundred amperes, the main current is advantageously converted into a comparatively weaker measuring current which is easy to handle, and which is passed through a measuring resistor. As a result the voltage drop across the measuring resistor is a measurement representing the value of the main current.
The known zero-flux current transformers are formed by a second harmonic magnetic modulator combined with a magnetic integrator. In principle a magnetic integrator includes a ring core of a ferromagnetic material, said ring core being provided with a primary winding, a secondary winding and a sensing winding. The sensing winding communicates with the inlet terminals of an amplifier driving the secondary current through a measuring resistor. Therefore an alteration of the magnetic flux in the ring core induces a voltage in the sensing winding, and this voltage effects the amplifier in such a manner that said amplifier generates a compensating current which counteracts the alteration of the flux caused by the primary current. As a result the magnetomotive forces generated by the current through the primary winding are equalized by the magnetomotive forces generated by the current trough the secondary winding in such a manner that a specific relationship exists between the current value in the primary winding and the current value in the secondary winding.
However, the magnetic integrator cannot handle DC and very low frequencies, and accordingly a separate circuit is required for this function. Such a circuit is formed by a magnetic modulator including two identically wound ring cores and a driver circuit. The ring cores are driven in saturation, and at a balance equaling zero flux the current curves are symmetrical in relation to zero. As a result the contents of equal harmonics are zero. Therefore it is possible to use either a direct, symmetrical detection by means of a window comparator/Schmitt trigger or a second harmonic detector using synchronous rectification of the current signal presenting the double frequency of the modulation signal. In both cases an output signal is obtained which is zero at a balance between the primary and the secondary ampere winding number. An imbalance in the ampere winding number results in a voltage of an amplitude and a polarity which depend on the degree and polarity of said imbalance. One core suffices for carrying out the latter detector function, but two cores in antiphase are necessary in order to prevent the compensating circuit from suppressing the detection and in order to prevent the modulation signal from interfering with the magnetic integrator at magnetic coupling.
Furthermore Danish Patent Application No. PA 2003 01752 discloses a detector circuit of the above kind. However, the circuit described therein is relatively complicated.